1. Field of the Invention
The present invention relates to a clear ice making apparatus and a clear ice making method for making a clear ice in a household refrigerator.
2. Related Art of the Invention
In conventional household refrigerators, to make a clear ice, an ice making tray is vibrated once water is poured into it, thereby preventing air bubbles produced during freezing from remaining in the resulting ice, or water having a dissolved gas such as air previously removed therefrom is used.
Alternatively, once water has poured into an ice making tray, the upper part of the ice making tray is heated to develop a temperature difference between the upper and lower parts of the ice making tray, thereby preventing air bubbles produced during freezing from remaining in the resulting ice.
Alternatively, in addition to avoiding air bubbles, to prevent hard ions such as calcium ions from being deposited in the resulting ice and thus making the ice cloudy, industrial refrigerators adopt a process in which an ice making tray in which water is to be frozen is set face down and water is supplied in the shape of a fountain into it, thereby gradually producing an ice on the side face of the ice making tray.
Alternatively, there is a process of producing a single crystal ice which is modeled on the process of production of a natural ice stalagmitic.
A large problem about making a clear ice is how to prevent air bubbles produced during freezing from being trapped in the resulting ice. Another problem is how to prevent hard ions contained in highly hard well water or mineral water in themselves from being deposited or air bubbles from being produced by impurities such as hard ions becoming cores thereof.
Specifically, general tap water contains about 15-30 ppm of hard ions and about 20 ppm of a dissolved gas. When freezing water, whether the resulting ice is clear or cloudy depends upon a combination of an interface shift rate of the solid-liquid interface between the ice and water (a rate of crystallization of water) and a diffusion rate of impurities ejected from the crystal (a rate of ejection of impurities from the ice). Therefore, in order to make a clear ice, it is essential to make an ice as slowly as possible, and thus, there is a problem in that the time required to make an ice cannot be shortened even if it is desired.
In particular, when the ice is cloudy because of dissolved air, diffusion of air in water is significantly involved. If the shift rate of the interface between the ice and water is high, the dissolved air remains in the ice. However, if the interface shift rate is low, molecules of air excluded from the ice are accumulated in water near the interface, thereby forming a region containing an excessive concentration of molecules of air. Such excessive molecules of air increase as the ice grows, and then, when the amount thereof go beyond a certain limit, the molecules form a macroscopic air bubble, which is eventually trapped in the growing ice.
In addition, the rate of ice making is also reduced by the temperature at the solid-liquid interface being increased by latent heat generated when a liquid phase changes into a solid phase at the static solid-liquid interface.
Even in the case where water is poured into an ice making tray at once, and the ice making tray is vibrated to prevent air bubbles from remaining in the resulting ice, when a large amount of water is frozen at once, the amounts of dissolved gas and hard ions contained in the water are large. Thus, the hard ions may be accumulated in the surface of the resulting ice to make the ice cloudy.
In the case of making an ice based on the principle of production of a natural ice stalagmitic, a single crystal ice of extremely high quality can be made. However, there is a problem in that the rate of ice making is extremely low, and it takes several days to make an ice.
Furthermore, the process of setting an opening of the ice making tray face down and suppling in the shape of a fountain into it involves a bulky apparatus and thus, is hot suitable for household application.
The process of physically vibrating the ice making tray to prevent air bubbles produced during crystallization of water from remaining in the resulting ice, a certain degree of transparency can be achieved. However, in the case where the air bubbles produced are small, there is a problem in that the bubbles are not separated from the interface between the ice and water and are trapped in the ice.
The process of removing a gas from water before crystallization is effective in making a clear ice. However, it involves a large-scale arrangement, resulting in a significant increase of cost. Furthermore, it has a problem in that if ice making takes a long time, air is dissolved again in the degassed water, air bubbles are produced during crystallization, and thus, an ice with a high transparency cannot be obtained.
Furthermore, there is a process of making a single crystal ice with a high transparency by dropping water droplets on a plane surface without a tray. However, the process has a problem in that, for the household and industrial refrigerators, it is required to make ices in a tray, and thus, an ice similar to the natural ice stalagmitic cannot be made.
As described above, conventional ice making apparatus have a problem in that it is difficult to make an ice with a high transparency.